1. Field of the Invention
This invention relates to a photographic material, preferably a photothermographic material, comprising photographic silver halide and a combination of components that enables any latent image silver in the material to be erased by a simple uniform heating step prior to imagewise exposure and development of the photographic material. The invention also relates to a process of producing an image by means of such a photographic material.
2. Description of the State of the Art
Photographic silver halide materials that are reversibly desensitized to light are known. These are described in, for example, U.S. Pat. No. 3,250,618. Such photographic silver halide materials are resensitized by heating the photographic material prior to imagewise exposure. Such photographic materials employ chemical desensitizers, such as mercuric chloride or triammonium hexachlororhodite, that may permanently adversely affect sensitometric properties of the photographic silver halide. These photographic materials do not comprise a combination of components that enable latent image silver to be heat erasable.
Photothermographic materials that comprise a source of halide ions for formation of photosensitive silver halide prior to exposure are also known. An example of such a photothermographic material that becomes photosensitive upon heating is described in U.S. Pat. No. 4,173,482. The photothermographic material does not contain photosensitive silver halide prior to heating, that is the photographic silver halide is not pre-formed. Such photothermographic materials in which the silver halide is formed in situ do not enable use of conventional silver halide emulsion making procedures and technology. Control of formation of silver halide grain size, shape and distribution as in conventional silver halide emulsion preparation is not possible.
Cobalt(III) amine salts are also known in imaging materials, such as described in U.K. Patent Specification No. 1,461,892. Up to now it was considered desirable to avoid cobalt(III) amine salts in preparing silver halide photographic materials because the cobalt(III) amine salts were thought to produce undesired desensitization. It has now been found that cobalt(III) amine salts can be desirable in photographic silver halide materials to enable the silver halide materials to be light handleable.
A problem has existed in providing photographic silver halide materials, such as photothermographic materials, that (1) are preparable and handleable in light prior to imagewise exposure and processing, (2) enable use of conventional silver halide emulsion preparation technology without the need for darkroom or safelight conditions in preparation, and (3) provide minimum density areas of an exposed and developed image which have reduced fog without the need for an added antifoggant. The art has not suggested an answer to this problem.
The art contains no suggestion that the answer to such a problem is in a heat erasable photographic material. The term "heat erasable photographic material" herein means a photographic silver halide composition or photographic silver halide element that has the capability, upon being heated in a heating step, of changing any latent image silver in the composition or element to a form which does not adversely affect imagewise exposure and development of the photographic material subsequent to the heating step. A latent silver image prior to development in which a photographic material is capable of being erased by heating the photographic material. No heat erasable photographic material has been suggested in the photographic art.